The invention relates to a method and system for recognizing the readiness for operation of an oxygen measurement sensor which is located in the exhaust pipe of an internal combustion engine and, together with a regulating device, serves to regulate the preparation of the mixture for the internal combustion engine.
In order to obtain exhaust gasses which are as free of injurious substances as possible, regulating devices for internal combustion engines are known in which the oxygen content in the exhaust pipe is measured and evaluated. For this purpose, oxygen measurement sensors, so-called lambda sensors, are known which operate in accordance with the principle of ion conduction through a solid electrolyte as a result of a difference in oxygen partial-pressure and give off, corresponding to the oxygen partial-pressure present in the exhaust gas, a voltage signal which exhibits a jump in voltage upon transition from an oxygen deficiency to an excess of oxygen.
The internal resistance of the known oxygen measurement sensors is, however, so large at low temperatures that the signal given off by the oxygen measurement sensor upon a cold start and during the warming-up phase of the internal combustion engine cannot be evaluated. In known devices for regulating the preparation of the mixture, a control which is independent of the output signal of the oxygen measurement sensor is therefore provided up to the region of readiness for operation of the oxygen measurement sensor. Only when the oxygen measurement sensor has reached its readiness for operation is its output signal used to regulate the fuel/air ratio.
In one known method of monitoring the readiness for operation of an oxygen measurement sensor, the oxygen measurement sensor is acted on by a test voltage having a constant mean value of the voltage which can be produced by the oxygen measurement sensor. The resultant voltage at the output of the sensor is fed as control variable of the readiness for operation to two comparison devices for comparison with an upper sensor value and a lower sensor value, in each case corresponding to a minimum output voltage of the oxygen measurement sensor, and via a timing element, a mixture control device is connected or disconnected instead of the mixture regulating device corresponding to the output signal of the comparison device.
In another known method of monitoring the readiness for operation of an oxygen measurement sensor in which a voltage resulting under the influence of the behavior of the sensor is also detected by two comparison circuits with threshold voltages and then evaluated, a constant reference voltage is connected in opposition for the detection of the sensor internal resistance of the oxygen measurement sensor which characterizes the readiness for operation of the sensor, in which connection the threshold voltages of the comparison circuits, which voltages are connected in opposition to the output voltage resulting therefrom, lie above and below the reference voltage by predetermined difference values, and in this connection the output signals from a subsequent evaluation circuit which are given off by the comparison circuits and correspond to a total of three logical states of switching, are processed for switching from regulation to control and vice versa.